Although immunotherapy of cancer is a promising approach for the treatment of cancer the enthusiasm for using this therapy is tempered by the fact that consistently successful treatment of patients is not achieved. A major mechanism impairing the optimal activation of an antitumor immune response is the promotion of networks of immune suppression by the tumor. T regulatory cells (Tregs) induce immune cell tolerance by directly inhibiting T cells, NK cells and dendritic cells. Compelling evidence obtained from both clinical and pre-clinical studies indicate that Tregs are increased in tumor bearing hosts and the accumulation of these cells in cancer patients is one of the main barriers for the optimal activation of an antitumor immune response. Depletion or inhibition of the suppressive function of Tregs enhances the antitumor immune responses and prolonged the survival of animals. However, currently there are no effective strategies to deplete or inhibit Tregs without affecting function of CD4 and CD8 T effector cells. We identified that neutralization/blockade of IL-9 with anti-IL-9 antibodies inhibits the suppressive function of Tregs without affecting the function of CD4 and CD8 T effector cells. Furthermore, the combination of tumor vaccination and anti-IL-9 induce tumor rejection in BALB-neuT and MUC-1 tolerant transgenic mice. These results led us to hypothesize that IL-9 play a role in Treg biology during the inflammatory (tumor growth) process enhancing/promoting the suppressive function of these cells and that the blockade of IL-9 could serve as a novel strategy to perturb the function of Tregs to enhance the antitumor effect of tumor vaccines. The objectives of this proposal are: Aim 1 will evaluate how IL-9 influences the function of Tregs; Aim 2 will optimize the neutralization/blockade of IL-9 for the induction of antitumor immune responses in BALB-neuT and MUC-1 tolerant transgenic mice; and Aim 3 will evaluate the effect of neutralizing/blocking IL-9 on human Tregs. The information of these studies will reveal new strategies for controlling and manipulating Tregs in order to enhance the antitumor immune response following tumor vaccination. .